Thermoplastic foam and process for the preparation thereof

ABSTRACT

Anhydride-containing thermoplastic polymers are foamed with ammonium carbonate, ammonium bicarbonate and ammonium oxalate to provide foams having good heat distortion characteristics and undesirable organic blowing agents are avoided.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 133,086, filed Mar. 24,1980, now abandoned.

Synthetic resins of thermoplastic foams in many instances are preparedby the extrusion of heat plastified gel containing organic blowingagents such as low boiling hydrocarbons or halogen containing lowboiling compounds such as methyl chloride and the various fluorinecontaining compounds. On extrusion of such blowing agents containingmobile gels into a region of lower pressure, the blowing agent causesthe gel to foam. At this period and shortly thereafter frequentlyblowing agent is released to the atmosphere thereby providing either aflammability hazard, if hydrocarbons are employed, or a halo compound isreleased to the atmosphere. Oftentimes such blowing agents act asplastifiers for the polymer in which they are used with the result thatless than desired heat distortion properties are obtained in theresultant foam. Generally in the extrusion of such thermoplastic foamswith organic volatile fluid blowing agents, it is necessary that themobile gel be prepared in a relatively complex equipment capable ofwithstanding high internal pressures.

It would be desirable if there were available an improved method for thepreparation of synthetic resinous thermoplastic foams.

It would also be desirable if there were available an improvedthermoplastic foam having desirable heat distortion characteristics.

It would also be desirable if there were an improved method for thepreparation of thermoplastic foam wherein no undesirable gases werereleased to the atmosphere.

These benefits and other advantages are obtained in a method for thepreparation of synthetic resins of thermoplastic foam wherein asynthetic resinous thermoplastic resin in admixture with a blowingagent, is heat plastified under pressure and subsequently extruded to azone of lower pressure wherein the blowing agent provides gas pressurewithin the gel forming a plurality of closed cells within the gel andsubsequently cooling the foamed mobile gel to a temperature at which itis below its heat plastification temperature, the improvement whichcomprises employing a major portion of the heat plastifiable resin, anethylenic addition polymer of mer units having the general formula##STR1## wherein A is at least one tetravalent organic radical having afirst pair of valences or vicinal carbon atoms attached to the carbonylgroups and a second pair of valences or vicinal carbon atoms formingpart of the polymer chain and arising by addition polymerization of aC═C ethylenic double bond in the monomeric form of A, i.e. anethylenically unsaturated dicarboxylic acid cyclic anhydride, B is thepolymerized mer unit corresponding to at least one ethylenicallyunsaturated addition polymerizable monomer, and x and y are numbers suchthat the ratio thereof represents the average proportion on a mole basisof the respective mer units in the polymer and the sum thereofrepresents the average degree of polymerization in the polymer, and asblowing agent a member selected from the group consisting of ammoniumcarbonate, ammonium bicarbonate, ammonium oxalate and mixtures thereof.

Also contemplated within the scope of the invention is an improvedresinous foam, the foam being prepared from an ethylenic additionpolymer of mer units having the general formula ##STR2## wherein A is atleast one tetravalent organic radical having a first pair of valences orvicinal carbon atoms attached to the carbonyl groups and a second pairof valences or vicinal carbon atoms forming part of the polymer chainand arising by addition polymerization of a C═C ethylenic double bond inthe monomeric form of A, i.e. an ethylenically unsaturated dicarboxylicacid cyclic anhydride, B is the polymerized mer unit corresponding to atleast one ethylenically unsaturated addition polymerizable monomer, andx and y are numbers such that the ratio thereof represents the averageproportion on a mole basis of the respective mer units in the polymerand the sum thereof represents the average degree of polymerization inthe polymer by heat plastifying polymer in the presence of a blowingagent selected from the group consisting of ammonium carbonate, ammoniumbicarbonate, ammonium oxalate and mixtures thereof, the foam containingmaleimide groups as determined by infrared spectroscopy.

Polymers suitable for the practice of the present invention are preparedfrom ethylenically unsaturated monomers the polymerization of whichprovide polymers that contain from about 2 to 30 weight percent of anethylenically unsaturated dicarboxylic acid cyclic anhydride.

The ethylenically unsaturated addition polymerizable monomers, fromwhich mer units B are derived, include aliphatic and aromatichydrocarbons and derivatives thereof having at least one polymerizableethylenic group, including olefinic monomers such as ethylene andpropylene, substituted olefinic monomers such as vinyl chloride, andvinyl acetate, styrenic monomers such as styrene, vinyltoluene, andmethylstyrene and ar-halostyrenes, acrylic monomers such as acrylicacid, methacrylic acid, ethyl acrylate and other alkyl acrylates, methylmethacrylate and other alkyl methacrylates and aryl acrylates andmethacrylates, acrylonitrile, methacrylonitrile, acrylamide and thelike.

The ethylenically unsaturated dicarboxylic acid cyclic anhydridemonomers, from which the mer units that include the radical A arederived, include those having the general formula ##STR3## wherein R ishydrogen, alkyl, aryl, halo and other substituent radicals. Illustrativeexamples of such anhydrides are:

maleic anhydride

citraconic anhydride

itaconic anhydride

phenyl maleic anhydride

tetrahydrophthalic anhydride

bicyclo(2.2.1)hept-5-ene-2,3-dicarboxylic acid anhydride

The preparation of such polymers is well known. Blowing agents suitablefor the practice of the present invention are ammonium carbonate,ammonium bicarbonate and ammonium oxalate and mixtures thereof. Thesolid blowing agents are at levels of from about 0.5 to 20 parts byweight/per hundred parts by weight of polymer (pph) and beneficiallyfrom 2 to 10 pph depending on the density of the foam desired. Suchblowing agents may be used alone or in conjunction with volatilefluorocarbons such as:

Fluorotrichloromethane

dichlorodifluoromethane

fluorodichloromethane

chlorodifluoromethane

chlorodifluoroethane

dichlorotetrafluoroethane

chloropentafluoroethane

difluoropropane

octafluorocyclobutane

The presence of such fluorocarbons within the cells results in reducedheat transfer through the foam plastic body. Foams in accordance withthe present invention are readily prepared using a screw extruder.Generally, it is desirable to dry blend or admix particulate polymerresin with a particulate blowing agent prior to addition to the screwextruder. Extrusion temperatures suitable for the preparation of foam inaccordance with the present invention are approximately the temperatureat which the polymer would be extruded if no blowing agent is employed.

The invention is illustrated but not limited to the following examples:

EXAMPLE 1

A dry blend was prepared by mixing the following ingredients at roomtemperature for 90 minutes in a Hobart Model N-50 mixer. 400 grams of astyrene maleic anhydride copolymer which was a 21.7 weight percentmaleic anhydride having a solution of viscosity of 4.97 centipoise as a10 weight percent of polymer dissolved in methyl ethyl ketone, 1.32grams of a resinous binder which is a 1 to 1 by weight mixture of a lowmolecular weight polystyrene and dibutyl phthalate, 4 grams of talc and20 grams of ammonium bicarbonate. The resultant blend was extrudedemploying a Brabender Type 2003, 3/4" screw extruder with the screwhaving a length/diameter ratio of 20 to 1. The peripheral temperature ofthe extruder is maintained at 210° C. and the die temperature 165° C.The die pressure is about 1000 pounds per square inch gauge. The die hadan aperture 1/4" in diameter. The resultant foam was white, uniform cellstructure and had a density of 2.4 pounds per cubic foot. A portion ofthe foam was analyzed by infrared spectroscopy and showed evidence ofthe maleimide structure.

EXAMPLE 2

Procedure of Example 1 was repeated with the exception that the 20 gramsof ammonium carbonate was replaced with 20 grams of ammoniumbicarbonate. The resultant foam had a desirable color, uniform cellstructure and a density of 9.3 pounds per cubic foot.

Similar beneficial results are obtained when ammonium oxalate is used inplace of ammonium carbonate or ammonium bicarbonate and the foamsobtained in accordance with the present invention have desirably higherheat distortion temperatures and improved solvent resistance.

As is apparent from the foregoing specification, the present inventionis susceptible of being embodied with various alterations andmodifications which may differ particularly from those that have beendescribed in the preceding specification and description. For thisreason, it is to be fully understood that all of the foregoing isintended to be merely illustrative and is not to be construed orinterpreted as being restrictive or otherwise limiting of the presentinvention, excepting as it is set forth and defined in thehereto-appended claims.

What is claimed is:
 1. An improved extruded closed-cell resinous foam,the foam being prepared from an ethylenic addition polymer consistingessentially of mer units having the general formula ##STR4## wherein Ais at least one tetravalent organic radical having a first pair ofvalences or vicinal carbon atoms attached to the carbonyl groups and asecond pair of valences or vicinal carbon atoms forming part of thepolymer chain and arising by addition polymerization of a C═C ethylenicdouble bond in the monomeric form of A, i.e. 2 to 30 weight percentbased on the weight of the polymer an ethylenically unsaturateddicarboxylic acid cyclic anhydride, B is the polymerized mer unitcorresponding to at least one ethylenically unsaturated additionpolymerizable monomer, and x and y are numbers such that the ratiothereof represents the average proportion on a mole basis of therespective mer units in the polymer and the sum thereof represents theaverage degree of polymerization in the polymer by heat plastifyingpolymer in the presence of about 0.5 to 20 parts by weight per 100 partsby weight of polymer of a blowing agent selected from the groupconsisting of ammonium carbonate, ammonium bicarbonate, ammonium oxalateand mixtures thereof, the foam containing maleimide groups as determinedby infrared spectroscopy.
 2. The foam of claim 1 wherein mer A is maleicanhydride.
 3. The foam of claim 1 wherein mer B is styrene.
 4. The foamof claim 1 wherein mer A is maleic anhydride, mer B is styrene and theblowing agent employed to form the foam is ammonium bicarbonate.